The invention relates to installations for protecting aircraft passengers against the risks of cabin depressurization during flights at high altitude.
Conventional installations for protecting passengers comprise a store of oxygen connected to a distribution unit which, in the event of the cabin depressurizing at high altitude, feeds oxygen at a pressure that increases with altitude to a distribution main having passenger masks connected thereto via respective flexible hoses formed with metering orifices. One such installation is described in document GB-A-828 362.
A protection installation is used infrequently. It must be made up of components that are lightweight, simple, cheap to make, and unlikely to break down. At present, the masks used in such installations are generally constituted by a shell of semi-flexible material which is fed continuously through an economizer bag into which the hose opens out. The economizer bag is connected to the inside of the face cover via an intake non-return valve. The face cover has a breath-out non-return valve fixed thereto and rated to maintain a pressure inside the face cover that is slightly higher than the pressure of the ambient atmosphere. While the user is breathing out, the intake valve is closed and the economizer or rebreather bag serves to collect the flow of oxygen provided by the source instead of allowing it to escape to the atmosphere.
The pressure of oxygen delivered by the distribution unit is selected as a function of the cross-sectional flow area of the metering orifices so as to provide the masks with a mean flow of oxygen that is sufficient to maintain the minimum oxygen partial pressure in the trachea as required by administrative regulations, and at any altitude.
That solution gives results that are acceptable up to altitudes of about 12 200 m. At higher altitudes, the present solutions do not provide effective protection against hypoxia. Unfortunately, airliners and business aircrafts are nowadays flying more and more frequently at altitudes exceeding 12 200 m, once they have lost weight due to consumption of part of their fuel.
At first sight, it would seem that satisfactory protection would require replacement of conventional installations of the type defined above with installations that include, for each mask, a demand regulator and a balanced breath-out valve (i.e. a valve that is insensitive to variations in downstream pressure), thereby enabling passengers to breathe pressurized pure oxygen by avoiding opening of the breath-out valve as soon as a determined pressure difference above ambient is reached. In practice, the cost and the complexity of such a solution would be prohibitive.